Do NOT send email likely to be flagged as spam by
SpamAssassin. I realize this
is the least deterministic filter I have, but it rarely gets a false
positive (or false negative ...). If you don't happen to hear from me, and
you followed all the other rules, send me a polite email asking me to check
my SA drop.

If your mail is rejected and has a "spam-report" included:

Do NOT quote your original message if you send me a notice to check the SA drop, as that may trigger SA again.

DO send me an e-mail with the contents of the "spam-report" if you
are unable to understand how to correct the issue.

DO pgp sign and/or pgp encrypt your message. By doing so, you
can bypass some filters.

If you don't do this, only use text/plain.

Do NOT send me email with a content-type of text/html. I can't read
this in my client, and it WILL be filtered out. Use format=flowed because it's cool.
one essay why

Do NOT send me email with a content-type of multipart/alternative.
There is no reason for this. Anything you can say,
you can say with text/plain. Considering the only thing I'll see anyway
is the text/plain portion, if you can't say it with text/plain, you wouldn't
have said it.

Do NOT send me attachments from an e-mail address that isn't whitelisted.
If we've exchanged more than a few emails, then you're probably alright.
Otherwise, sign/encrypt your entire message with PGP before sending it.

Use a client that uses 'In-reply-to' headers (notably Exchange Server and its clients behaves poorly), and use the 'reply' function correctly.

Do NOT use excessively long subject lines. If the entire body of your
message is the (short) subject line, indicate so (e.g. a subject of
"Will be home at 19:00 [NT]" fits these criteria). The tag [NT] is usually used for these purposes and stands for 'No Text'. Also, with the same usage, is "<EOM>", for "End of Message".

Sylpheed and Claws are excellent clients.
Eudora and
Pegasus Mail are both excellent alternatives as well.
I hear Opera also has an excellent client
included with their web browser.